Heteronuclear refocusing by nonlinear phase and amplitude modulation on a single transmitter channel.

The application of low magnetic fields to heteronuclear NMR has expanded recently alongside the emergence of methods for achieving near unity polarization of spin ensembles, independent of magnetic field strength. The parahydrogen induced hyperpolarization methods in particular, often use a hybrid arrangement where a high field spectrometer is used to detect or image polarized molecules that have been conjured on a separate, dedicated polarizer instrument operating at fields in the mT regime where yields are higher. For controlling polarizer chemistry, spare TTL channels of portable NMR spectrometers can be used to pulse program reaction timings in synchrony with heteronuclear RF transformations. The use of a spectrometer as a portable polarizer control module has the advantage of allowing detection in situ, simplifying the process of optimizing polarization yields prior to in vivo experimental trials. Suitable heteronuclear spectrometers compatible with this application are becoming more common, but are still sparsely available in comparison to a large existing infrastructure of single channel NMR consoles. With the goal of expanding the range of these systems to multinuclear applications, the feasibility of rotating a pair of heteronuclear spins ((13)C and (1)H) at 12mT was investigated in this study. Nonlinear phase and amplitude modulated waveforms designed to simultaneously refocus magnetization at 128kHz ((13)C) and 510kHz ((1)H) were generated numerically with optimal control. Although precise quantitative comparisons were not attempted due to limitations of the experimental setup, signals refocused at heteronuclear frequencies with this PANORAMIC approach (Precession And Nutation for Observing Rotation At Multiple Intervals about the Carrier) yielded amplitudes comparable to signals which were refocused using traditional block pulses on heteronuclear channels. Using this PANORAMIC approach to heteronuclear NMR at low field would reduce expense as well as hardware complexity and bulk, weighed against the caveat that elaborate pulses are required. More work will be necessary to test this method on the targeted application of parahydrogen induced hyperpolarization as well as to quantify efficiency, but upon further development we anticipate that this method may offer a viable 'software' approach to heteronuclear manipulations of spins at low magnetic fields.

Prospective real-time head motion correction using inductively coupled wireless NMR probes.

PURPOSE: Head motion continues to be a major source of artifacts and data quality degradation in MRI. The goal of this work was to develop and demonstrate a novel technique for prospective, 6 degrees of freedom (6DOF) rigid body motion estimation and real-time motion correction using inductively coupled wireless nuclear magnetic resonance (NMR) probe markers. METHODS: Three wireless probes that are inductively coupled with the scanner's RF setup serve as fiducials on the subject's head. A 12-ms linear navigator module is interleaved with the imaging sequence for head position estimation, and scan geometry is updated in real time for motion compensation. Flip angle amplification in the markers allows the use of extremely small navigator flip angles (∼1°). A novel algorithm is presented to identify marker positions in the absence of marker specific receive channels. Motion correction is demonstrated in high resolution 2D and 3D gradient recalled echo experiments in a phantom and humans. RESULTS: Significant improvement of image quality is demonstrated in phantoms and human volunteers under different motion conditions. CONCLUSION: A novel real-time 6DOF head motion correction technique based on wireless NMR probes is demonstrated in high resolution imaging at 7 Tesla.

Highly-accelerated Bloch-Siegert |B1+| mapping using joint autocalibrated parallel image reconstruction.

PURPOSE: To reconstruct accurate single- and multichannel Bloch-Siegert transmit radiofrequency (|B(1)(+)|) field maps from highly accelerated data. THEORY AND METHODS: The approach is based on the fact that the |B(1)(+)|-to-phase encoding pulse for each transmit coil and off-resonance frequency applies a unique phase shift to the same underlying image. This enables joint reconstruction of all images in a Bloch-Siegert acquisition from an augmented set of virtual receive coils, using any autocalibrated parallel imaging reconstruction method. RESULTS: Simulations with an eight channel transmit/receive array head coil at 7T show that accurate |B(1)(+)| maps can be produced at acceleration factors of 16× and 6× for Cartesian and spiral sampling, respectively. A phantom experiment with a six channel transverse electromagnetic (TEM) transceive array coil allowed accurate reconstruction at 16× acceleration. 7T in vivo experiments performed using 32 channel receive and two-channel transmit coils further demonstrate the proposed method's ability to produce high-quality |B(1)(+)| maps at accelerations of 32× and 8× for Cartesian and spiral trajectories, respectively. Reconstruction accuracy is improved using disjoint k-space sampling patterns between acquisitions. CONCLUSION: The proposed approach allows high acceleration factors in Bloch-Siegert |B(1)(+)| mapping and can significantly reduce the scan time requirements for mapping the |B(1)(+)| fields of transmit arrays.

Double tuning a single input probe for heteronuclear NMR spectroscopy at low field.

Applications of PASADENA in biomedicine are continuing to emerge due to recent demonstrations that hyperpolarized metabolic substrates and the corresponding reaction products persist sufficiently long to be detected in vivo. Biomedical applications of PASADENA typically differ from their basic science counterparts in that the polarization endowed by addition of parahydrogen is usually transferred from nascent protons to coupled storage nuclei for subsequent detection on a higher field imaging instrument. These pre-imaging preparations usually take place at low field, but commercial spectrometers capable of heteronuclear pulsed NMR at frequencies in the range of 100 kHz to 1 MHz are scarce though, in comparison to single channel consoles in that field regime. Reported here is a probe circuit that can be used in conjunction with a phase and amplitude modulation scheme we have developed called PANORAMIC (Precession And Nutation for Observing Rotations At Multiple Intervals about the Carrier), that expands a single channel console capability to double or generally multiple resonance with minimal hardware modifications. The demands of this application are geared towards uniform preparation, and since the hyperpolarized molecules are being detected externally at high field, detection sensitivity is secondary to applied field uniformity over a large reaction volume to accommodate heterogeneous chemistry of gas molecules at a liquid interface. The probe circuit was therefore configured with a large (40 mL) Helmholtz sample coil for uniformity, and double-tuned to the Larmor precession frequencies of (13)C/(1)H (128/510 kHz) within a custom solenoidal electromagnet at a static field of 12 mT. Traditional (on-resonant) as well as PANORAMIC NMR signals with signal to noise ratios of approximately 75 have been routinely acquired with this probe and spectrometer setup from 1024 repetitions on the high frequency channel. The proton excitation pulse width was 240 µs at 6.31 W, compared to a carbon-13 pulse width of 220 µs at 2.51 W. When PANORAMIC refocusing waveforms were transmitted at a carrier frequency of 319 kHz, integrated signal intensities from a spin-echo sequence at both proton (510 kHz) and carbon-13 (128 kHz) frequencies were within experimental error to block pulse analogs transmitted on resonance. We anticipate that this probe circuit design could be extended to higher and lower frequencies, and that when used in conjunction with PANORAMIC phase and amplitude modulated arrays, will enable low field imaging consoles to serve as multinuclear consoles.